1. Field of the Invention
The present invention relates to a projector which employs an AC-driven ultra-high pressure mercury lamp as a light source lamp, and more particularly to a method of controlling the light source lamp for driving.
2. Description of the Related Art
FIG. 1 illustrates the configuration of a conventional projector which comprises an AC-driven light source lamp. As illustrated in FIG. 1, this conventional projector comprises RGB/video image processing circuit 10, scaler circuit 11, fixed pixel panel driving circuit 12, projection lens 13, fixed pixel panel 14 such as a transmission-type liquid crystal panel, a reflection-type liquid display panel, DMD (Digital Micromirror Device) or the like, projector main control circuit 45, AC-driven ultra-high pressure mercury lamp 19, lamp driving circuit 18 for driving ultra-high pressure mercury lamp 19, and lamp driving control circuit 47 for controlling ultra-high pressure mercury lamp 19 for driving through lamp driving circuit 18.
RGB/video image processing circuit 10 converts an analog video signal applied thereto to a digital video signal which is delivered therefrom. Scaler circuit 11 converts the resolution of the digital video signal from RGB/video processing circuit 10 to conform to the number of pixels on fixed pixel panel 14.
Fixed pixel panel driving circuit 12 drives fixed pixel panel 14 based on the video signal, the resolution of which has been converted by scaler circuit 11. Projection lens 13 includes a projection optical system for projecting transmitted light which has been transmitted by fixed pixel panel 14 onto screen 9.
Projector main control circuit 45 controls the overall projector. Projector main control circuit 45 in this conventional projector notifies lamp driving control circuit 47 of a driving frequency for fixed pixel panel 14.
In this conventional projector, fixed pixel panel 14 is driven based on a video signal applied thereto, and incident illumination light from ultra-high pressure mercury lamp 19, which serves as a light source lamp, is transmitted or reflected by fixed pixel panel 14 to project an image onto the screen.
Here, applied video signals may have vertical synchronizing signals at a variety of frequencies (hereinafter called the “vertical synchronizing frequency”). Specifically, the projector may be applied with video signals having a variety of vertical synchronizing frequencies, including video signals such as NTSC-, PAL-, SECAM-based video signals (vertical synchronizing frequency: 50/60 Hz), composite signals (525 i/p, 625 i/p, 720 i/p, 1080 i/p, vertical synchronizing frequencies: 25/30/4850/7-Hz) and the like, video signals from a personal computer (vertical synchronizing frequency: 50-120 Hz), and so forth.
Here, when the driving frequency for fixed pixel panel 14 is not synchronized to the driving frequency for ultra-high pressure mercury lamp 19, the driving frequencies interfere with each other to cause scrolling flicker (light, dark) to appear on projected image 9 displayed on the screen. To prevent such flicker, JP-A-2003-307721 has proposed a projector which sets a driving frequency for ultra-high pressure mercury lamp 19 in lamp driving control circuit 47 such that a driving frequency for fixed pixel panel 14 is in synchronization with the driving frequency for ultra-high pressure mercury lamp 19.
In the conventional projector disclosed in JP-A-2003-307721, the driving frequency for ultra-high pressure mercury lamp 18 is controlled to be an integer multiple of the driving frequency for fixed pixel panel 14 to establish the synchronization of the driving frequency for fixed pixel panel 14 with the driving frequency for ultra-high pressure mercury lamp 19.
For example, when a PAL video signal is applied to the projector to set the driving frequency for fixed pixel panel 14 to 50 (Hz), the driving frequency for ultra-high pressure mercury lamp 19 is set to 150 (Hz) or 200 (Hz) which is in synchronization with the driving frequency for fixed pixel panel 14. On the other hand, when an NTSC video signal is applied to the projector to set the driving frequency for fixed pixel panel 14 to 60 (Hz), the driving frequency for ultra-high pressure mercury lamp 19 is set to 180 (Hz) or 240 (Hz) which is in synchronization with the driving frequency for fixed pixel panel 14.
JP-A-2003-156798 in turn has proposed a projector which synchronizes the driving frequency for fixed pixel panel 14 with the driving frequency for ultra-high pressure mercury lamp 19 when the synchronization is possible, and sets a highest possible frequency at which scroll noise occurs due to the out-of-synchronization between the two driving frequencies. By thus increasing the frequency at which the scroll noise occurs, the resulting flicker does not seem flicker to human's eyes due to the human's visual characteristics.
For example, a description will be made of an exemplary situation in which the driving frequency for ultra-high pressure mercury lamp 19 is restricted within a range of 140-260 (Hz), and an applied video signal has a vertical synchronizing frequency of 87 Hz or higher. Also, assume that there is a condition that ultra-high pressure mercury lamp 19 is driven at 180 Hz or higher because a reduction in the driving frequency for ultra-high pressure mercury lamp 19 can result in flicker. Thus, when the driving frequency for fixed pixel panel 14 is set at 87 Hz, a frequency which is an integer multiple of 87 Hz cannot be set within the range of 180 to 260 Hz. To cope with this inconvenience, JP-A-2003-156798 discloses that in such a case, the driving frequency for ultra-high pressure mercury lamp 19 is set at 260 Hz which is the highest frequency within the restricted range to make the resulting flicker as inconspicuous as possible.
Next, FIG. 2 shows a lamp driving timing in the conventional projector. Here, since an applied video signal has a vertical synchronizing frequency of 60 Hz, and the driving frequency for fixed pixel panel 14 is at 60 Hz as well, lamp driving control circuit 17 sets the driving frequency for ultra-high pressure mercury lamp 19 at 180 (Hz). In this event, no scrolling flicker occurs because the driving frequency of 60 Hz for fixed pixel panel 14 is in synchronization with the driving frequency of 180 Hz for ultra-high pressure mercury lamp 19. However, even in this event, since the ultra-high pressure mercury lamp 19 is driven at a constant frequency, unscrolling flicker (light, dark) can occur when a video signal level is low, as represented by three vertical lines in FIG. 3.
While such unscrolling flicker is less conspicuous as compared with scrolling flicker, it can be pronounced as the case may be because it occurs at the same location. In addition, the unscrolling flicker can be perceived under bad conditions such as a low video signal level.
Also, the control for bringing the driving frequency for ultra-high pressure mercury lamp 19 into synchronization with the driving frequency for fixed pixel panel 14 involves following a change in the vertical synchronizing frequency of an applied video signal to change the driving frequency for ultra-high pressure mercury lamp 19. For this reason, if an applied video signal suffers from an instable vertical synchronizing frequency, the driving frequency for ultra-high pressure mercury lamp 19 follows the instability, resulting in failure in stably driving ultra-high pressure mercury lamp, judder of driving transformer, and a degraded reliability of the ultra-high pressure mercury lamp. Also, while the driving frequency for ultra-high pressure mercury lamp 19 can be set in a limited range, for example, from 140 to 260 (Hz) as described above, there is an optimal driving frequency at which the lifetime of ultra-high mercury lamp 19 can be made the longest. However, since the conventional projector changes the driving frequency based on the frequency of an applied video signal, the resulting driving frequency may not be optimal for the lamp. This can lead to a degraded reliability.